Other mechanisms, such as galaxy merger and
interaction, are also expected to displace the angular
momentum of the gas and transport the gas inward
[1]}, [2]}, [3]}.
Similar to the studies of secular evolution, observational
evidence for this scenario is also mixed.
Some studies found significant environmental dependence of AGN activities
[4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]},
while others found no or only weak environmental effects [15]}, [16]}, [17]}, [18]}, [19]}, [20]}, [21]}, [22]}, [23]}.
The contradictory results
may be caused by the difference in AGN selection criterion,
observational bias, control sample and environmental indicator used.
As we will show below, understanding environmental effects
on AGNs also requires the knowledge about the evolutionary status of
their host galaxies, as it can help us to better understand
how to construct control samples and to adopt appropriate
environmental indicators.
